Polymeric weathershed surge arrester and method

ABSTRACT

An elastomeric surge arrester housing includes a sleeve having a tubular core and radiating sheds. The sleeve is molded in a first configuration and, when the core is radially stretched, assumes a second configuration in which the sheds assume a downwardly extending configuration. The sheds, in the second configuration, include an upper surface having a generally frustoconical shape. The housing may be made using conventional molding techniques, but requires substantially less material than if the housing were molded directly into the second configuration.

This application claims the benefit of the U.S. Provisional No.60/012,637 filed Mar. 1, 1996.

BACKGROUND OF THE INVENTION

The present invention relates generally to electrical power distributionequipment. More particularly, the invention relates to surge arresters.Still more particularly, the invention relates to surge arrestersemploying polymeric weathersheds.

Under normal operating conditions, electrical transmission anddistribution equipment is subject to voltages within a relatively narrowrange. Due to lightning strikes, switching surges or other systemdisturbances, portions of the electrical network may experiencemomentary or transient voltage levels that greatly exceed the levelsexperienced by the equipment during normal operating conditions. Leftunprotected, critical and costly equipment such as transformers,switching apparatus, computer equipment, and electrical machinery may bedamaged or destroyed by such over-voltages and the resultant currentsurges. Accordingly, it is routine practice to protect such apparatusfrom dangerous over-voltages through the use of surge arresters.

A surge arrester is a protective device that is commonly connected inparallel with a comparatively expensive piece of electrical equipment soas to shunt or divert the over-voltage-induced current surges safelyaround the equipment, thereby protecting the equipment and its internalcircuitry from damage. When caused to operate, a surge arrester forms acurrent path to ground having a very low impedance relative to theimpedance of the equipment that it is protecting. In this way, currentsurges which would otherwise be conducted through the equipment areinstead diverted through the arrester to ground.

Conventional surge arresters typically include an elongate outer housingmade of an electrically insulating material, a pair of electricalterminals at opposite ends of the housing for connecting the arresterbetween a line-potential conductor and ground, and an array ofelectrical components in the housing that form a series path between theterminals. These components typically include a stack of voltagedependent, nonlinear resistive elements. These nonlinear resistors or“varistors” are characterized by having a relatively high resistance atthe normal steady-state voltage and a much lower resistance when thearrester is subjected to transient over-voltages. Depending on the typeof arrester, it may also include one or more electrodes, heat sinks orspark gap assemblies housed within the insulative housing andelectrically in series with the varistors.

To ensure proper operation of the arrester, the varistors and otherinternal components must be isolated from moisture and environmentalpollutants. The arrester housing serves to seal the components from theambient environment. Additionally, most surge arrester housings include“skirts” or “weathersheds” spaced apart along the length of the housing.An arrester, once installed outdoors, will be exposed to contaminants orenvironmental pollutants that are deposited on the housing surface byrain, wind and other conditions. These contaminants, over time, maybuild up to such a degree that they form a path for current. Suchbuildup effectively reduces the distance between energized orline-potential components and ground. In this manner, the surfaceresistivity of the arrester housing will decrease to a point whereflashover may occur and a short circuit result. Accordingly,weathersheds have traditionally been included on an arrester housing toextend or lengthen the housing surface and increase the effectivedistance between the energized arrester terminal and ground.Additionally, weathersheds have been designed to enhance the ability ofthe arrester to resist or to minimize the degree to which dust andenvironmental contaminants may build up on the housing's outer surface.Such designs have included varying the radii of adjacent sheds, usingparticularly designed materials that resist the effects ofcontamination, and by varying the number and size of the sheds on thehousing.

Surge arrester housings made of porcelain were once the industrystandard. Unfortunately, such arrester housings were fragile andfrequently were the subject of vandalism. Additionally, the porcelainhousing was heavy, requiring a substantial support means to mount thearrester. Furthermore, when a porcelain housed arrester failed, it wasnot uncommon for the housing to explode, sending porcelain fragments athigh velocities in all directions. Such failures presented the obviouspotential for danger to personnel and damage to equipment.

Presently, at least in distribution class surge arresters, a polymerichousing has become a standard feature. A polymeric housing is lessexpensive to manufacture, is nonfragmenting and is less susceptible todamage during shipment, installation and use compared to prior artporcelain housings. Additionally, a polymeric housing is substantiallylighter, allowing simpler and less costly installation.

The polymeric arrester housing is typically molded of silicone rubber oranother elastomeric material. The housing includes a central core andradiating sheds or skirts which are molded integrally with the centralcore. The central core includes an internal bore or chamber that issubstantially the same diameter as the varistors and other arrestercomponents to be housed therein. Where a particular shape or orientationof the sheds is desired, the mold for the housing is manufactured so asto provide that desired configuration.

Present molding techniques effectively limit the configuration andarrangement of sheds on a polymeric arrester housing. Further, becauseof limitations in the molding process, manufacturing housings withcertain weathershed orientations is costly and difficult. Also, thepresent methods of obtaining a good bond between the inside surface ofthe housing and the internal components is expensive and generates asubstantial amount of scrap material.

Accordingly, there remains a need in the art for a polymeric arresterhousing having an enhanced weathershed design that will resist buildupof environmental pollutants and, at the same time, is relatively simpleto manufacture using conventional molding techniques. It would furtherbe advantageous if the housing provided a superior bond between theinside surface of the housing and the internal electrical components.Given the present cost of silicone rubber and other elastomericmaterials known to be employed in arrester housings, it would be furtheradvantageous if the weathershed could be manufactured using lessmaterial than presently employed for similar housings.

SUMMARY OF THE INVENTION

The present invention includes an elastomeric housing for a surgearrester that includes a deformable shedded sleeve with a tubular corehaving central bore and a plurality of axially-spaced sheds radiallyextending from the core. The sleeve has a first configuration when thecore is unstretched, and a second configuration when the core isstretched. When the core is stretched radially, the sheds assume a newconfiguration in which the upper surface is generally frustoconical andin which the ends of the sheds move axially from their initialconfiguration; however, the ends of the sheds remain at the samepredetermined radial position in both the first and secondconfiguration. It is preferred that the sheds extend downwardly from thecore at an angle within the range of approximately 10 to 60°, and morepreferably 10 to 45°, when the sleeve is in the stretched configuration.

The elastomeric housing is preferably made of a silicon rubber and ismolded in the first, unstretched configuration. In that configuration,the upper surface of the shed joins the core portion in a shoulderhaving a radius of curvature of R₁ and the lower surface of the shedjoins the core portion in a lower shoulder having a radius of curvatureR₂, R₁ being greater than R₂. Additionally, in the first configuration,the upper surface of the shed includes a first transition point wheretwo frustoconical surface segments are joined. Also, in the firstconfiguration, the lower surface of the shed includes a secondtransition point at the intersection of a pair of frustoconical surfacesegments. The frustoconical surface segments on the upper surface taperdownwardly while the frustoconical surface segments on the lower surfacetaper upwardly. The sheds are configured such that the second transitionpoint is closer to the axis of the housing than the first transitionpoint. In addition, the downward angle on the top side is preferablygreater than or equal to the upward angle on the bottom side.

The present invention permits an elastomeric arrester housing to becreated with appropriately configured, downwardly extending sheds, butallows the housing to be molded with sheds that are substantiallyperpendicular to the axis of the housing. This provides significantmanufacturing advantages in that it is a much simpler process to mold anelastomeric housing having sheds that extend substantially perpendicularto the housing axis. Additionally, the invention permits an elastomerichousing that may be stretched or deformed so as to have a particularlyadvantageous configuration of downwardly extending sheds where thehousing is manufactured using significantly less volume of elastomericmaterial than if the housing were molded into the ultimately-desiredconfiguration using conventional techniques. These and various othercharacteristics and advantages of the present invention will be readilyapparent to those skilled in the art upon reading the following detaileddescription in referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For an introduction to the detailed description of the preferredembodiments of the invention, reference will now be made to theaccompanying drawings, wherein:

FIG. 1 is an elevational view, partially cutaway and partially incross-section, showing the surge arrester and arrester housing of thepresent invention;

FIG. 2 is a cross-sectional view of the arrester housing shown in FIG.1;

FIG. 3 is a cross-sectional view of the housing shown in FIG. 2 in itsas-molded and unstretched configuration;

FIG. 4 is an enlarged view of a portion of the as-molded and unstretchedhousing shown in FIG. 3;

FIG. 5 is a view similar to that shown in FIG. 4 showing across-sectional view of a portion of the weathershed both before andafter it has been stretched to accommodate and house the arrestercomponents shown in FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

It will be understood that the following components are representativeof the contexts in which the present invention can be used and are notintended to be an exhaustive identification thereof. Referring first toFIG. 1, surge arrester 10 and arrester housing 20 of the presentinvention are shown. Arrester 10 generally comprises hanger 12, top andbottom terminal studs 14, 16, ground lead disconnector 18 andelastomeric housing 20. Arrester 10 is supported by arrester hanger 12which, in turn, is mounted to a utility pole or other support member(not shown). Housing 20 encloses an array 22 of arrester componentswhich is an electrical device that are maintained in stacked end-to-endarrangement by an insulative component retention means 28. Retentionmeans 28 may comprise, for example, an insulative liner such as thatshown in U.S. Pat. No. 4,930,039 or filament windings such as disclosedin U.S. Pat. Nos. 5,138,517, 4,656,555 or 5,043,838. It is preferred,however, that insulative component retention means 28 be made in theform of a hardened resinous coating, reinforced with glass fibers, andhaving a coefficient of thermal expansion that is greater than thecoefficient of thermal expansion of the electrical components in array22 so as to provide an axial load on the components once cured andcooled. Such an embodiment is described in copending U.S. applicationentitled Self-Compressive Surge Arrester Module and Method of MakingSame, Ser. No. 60/012,667, filed Mar. 1, 1996, the entire disclosure ofwhich is incorporated herein by this reference.

Array 22 includes electrodes 25, metal oxide varistors (MOV's) 26 andend terminals 24 at each end. Upper and lower conducting studs 14, 16threadedly engage central threaded bores (not shown) in the ends ofterminals 24 so as to provide a means for connecting line potential andground lead conductors (not shown) to arrester 10. Conventional groundlead disconnector or isolator 18 is disposed about terminal stud 16 toprovide a means to explosively disconnect the ground lead in the eventof arrester failure. MOV's 26 are stacked within array 20 in end-to-endrelationship with electrodes 25 disposed between facing surfaces ofadjacent MOV's 26. MOV's 26 may be in the form of any conventionallyavailable metal oxide varistor. Although not shown in FIG. 1, array 22may also include a variety of other electrical components, includingheat sink or spacer elements or spark gap assemblies which maythemselves include ceramic materials, such as silicon carbide ringshaving voltage dependent resistances.

Housing 20 is best shown in FIG. 2. Housing 20, as shown, has particularutility when employed in a distribution class surge arrester. Althoughthe principles of the present invention may be employed in surgearresters having other physical dimensions and ratings, the inventionwill be understood and will be described herein with reference to the 10KA heavy duty 10 KV (8.4 KV MCOV) distribution class arrester shown inFIG. 1.

Referring still to FIG. 2, housing 20 generally comprises a sleevehaving a central tubular core 30 and downwardly extending sheds 36attached to core 30 in axially spaced apart relation. Housing 20 maytherefore be described as a shedded sleeve. Core 30 includes centralbore 31, inner cylindrical surface 32 and outer cylindrical surface 34.Sheds 36, which are integrally molded with core 30, extend from outersurface 34 and include an upper surface 38, lower surface 40 and outeredge 42. Upper and lower surfaces 38, 40 are generally frustoconical inshape although, as described more fully below with reference to FIG. 5,surfaces 38 and 40 each include certain segments 61, 63 that are concaveand other segments 62 that are convex. Sheds 36 extend radially outwardfrom core 30 and preferably are inclined between approximately 10 and60°, and more preferably between 20 and 45°, from a plane perpendicularto the central axis of housing 20. This angle of inclination indicatesthe angle of the greater top shed surface 38.

The inclined shed shape has several advantages. The inclined angleassures that a portion of the shed is protected from both contaminationand wetting such that it maintains a high surface resistivity. Theremaining surface can become contaminated with salts and dust and willhave a much lower surface resistivity when wet, but the inclination willtend to wash much of the contamination off.

Still referring to FIG. 2 in its as-used configuration, core 30 includesan inside diameter D₁ measured from opposite sides of inner cylindricalsurface 32 and an overall outer diameter D₂ as measured from oppositeshed ends 42 as shown in FIG. 2. In this embodiment, D₁ is substantiallyequal to 1.7 inches and D₂ substantially equal to 3.6 inches. Housing 20is molded from an elastomeric material to enable the housing to bestretched as described more fully below. Preferably, housing 20 is madeof polymeric material, such as silicone rubber. To permit the stretchingand deformation required, housing 20 should be made from a siliconerubber. While other elastomeric compounds can be used, silicone ispreferred because of its natural resistance to UV radiation. Althoughother compounds can be formulated to resist UV degradation, some surfacedamage will still occur, increasing the risk of tear propagation fromsurface flaw sites. The advantage of using silicone to form the housinglies in the ability of silicone to repel water. When water full ofcontaminants beads up on the surface, the surface resistivity is muchhigher than if the water were present as a surface wetting film. Othermaterials have provided a hydrophobic quality when new, but lose thistrait as they age. Suitable materials for housing 20 are those suppliedby Dow Corning STI, General Electric Silicones, Wacker Silicones,DuPont, and Uniroyal, and having elongation at break per ASTM D412higher than the stretched elongation levels and also exhibiting goodphysical and electrical performance for their operating environment perwell known industry standards. The preferred polymer system is a highlyfilled silicone system containing Aluminum Trihydrate (“ATH”) surfacetreated fumed silica and optional extending fillers such as silicaflour. This system preferably has an elongation at break of greater than300%, a durometer (shore A) of less than 50, and a Wet Arc Trackperformance of 180 minutes at 6 kV when the sample is tested atstretched level approximately 125% of the level in the application. Anadditional desirable criteria is for the failure mode after Wet ArcTrack Testing to be nontracking in nature, i.e., due to materialerosion, and such that there is no evidence of tear propagation at thefailure site. If these conditions are met, the housing will continue towithstand voltage and extend product life, even after a localizedmaterial failure has occurred.

Referring now to FIG. 3, housing 20 is shown in its as-moldedconfiguration, prior to it being stretched and deformed into its as-usedconfiguration so as to accommodate MOV's 26 and the other arrestercomponents of array 22 which is an electrical device. In thisunstretched configuration, sheds 36 are axially spaced apartapproximately 1.375 inches and core 30 has an inside diameter of D₁′ andan outside diameter D₂′. In its unstretched configuration, D₁′ isapproximately 1.2 inches, or 60 to 90% of D₁. Importantly, however, theoutside diameter D₂′ of the unstretched housing 20 is substantially thesame as the overall diameter D₂ of housing 20 when stretched. To achievethe desired configuration of housing 20 as shown in FIG. 2 when insidediameter D₁′ is increased to D₁, it is important that housing 20 and,particularly, sheds 36 be molded to have particular inclinations andradii of curvature and degrees of taper. More specifically, andreferring now to FIG. 4, upper surface 38 of shed 36 joins outer surface34 of core 30 at upper arcuate surface 46. The terms “upper” and “lower”are used hereinafter to refer to relative positions and orientations asshown in the figures. Upper arcuate surface 46 (first shoulder) has aradius of curvature designated as L, R₁ which, in the embodiment shownis substantially equal to 0.375 inches. Similarly, lower surface 40 ofshed 36 intersects core outer surface 34 at lower arcuate surface 48(second shoulder), which has a radius of curvature equal to R₂. In thisembodiment, R₂ is substantially equal to 0.093 inches. Without regard tothe precise radii, to achieve the desired change in inclination andshape of weathersheds 36 from that shown in FIG. 1 to that shown in FIG.2, R₁ should be greater than R₂ and is preferably at least twice asgreat as R. In addition, the downward angle on the top side ispreferably greater than or equal to the upward angle on the bottom side.

Referring still to FIG. 4, upper and lower surfaces 38, 40 each includea pair of frustoconical segments having varying degrees of incline ordecline as measured from a plane that is substantially perpendicular tothe longitudinal axis of housing 20. These frustoconical segments arebest described with reference to transition points 51-54. As molded,shed 36 includes an upper surface comprising first and second upperfrustoconical segments 55, 56 and a lower surface 40 comprising firstand second lower frustoconical segments 57, 58. First upperfrustoconical surface segment 55 extends between transition point 51 andtransition point 52 and slopes downwardly at an incline from horizontalequal to α₁. Second upper frustoconical surface segment 56 extends fromtransition point 52 to shoulder 59 adjacent outer edge 42, and tapersdownwardly at an angle from horizontal equal to α₂. First lowerfrustoconical surface segment 57 extends between transition points 53and 54 and inclines upwardly from the horizontal at an angle equal toα₃. Second lower frustoconical surface segment 58 extends betweentransition point 54 and outer edge 42 and is inclined upward from thehorizontal at an angle equal to α₄. α₁-α₄ will vary depending upon thesize of housing 20 and the precise operational orientation desired ofsheds 36, however, for the embodiment shown in FIG. 1, for example,α₁-α₄ will have the following values.

Angle Degrees α₁ 10° α₂  1° α₃  0.5° α₄  0.5°

Without regard to the precise values of α₁-α₄, according to theinvention, transition point 51 should be at a greater radius from theaxis 21 of housing 20 than transition point 53, and transition point 52should be at a greater radius than transition point 54. In the specificembodiment described herein, transition point 52 is located at a radialdistance substantially equal to 1.467 inches, while transition point 54is located at a radial distance substantially equal to 1.342 inches.Also in this embodiment, transition point 51 is located at a radialdistance substantially equal to 0.37 inches and transition point 53 islocated at a radial distance substantially equal to 0.09 inches.

In some instances (not shown), it may be preferred to use only a singlefrustoconical section for lower surface 40. This surface extends from asingle transition point, with that single transition point being betweenthe two transition points 51, 52 on upper surface 38.

In its unstretched configuration as shown in FIGS. 3 and 4, housing core30 has a wall thickness of substantially 0.109 inches and outer edge 42is approximately is 1.090 inches from outer surface 34 of core 30 sothat D₂′ equals approximately 3.614 inches. D₁′ is substantially equalto 1.216 inches.

Upon assembly of arrester 10, MOV's 26 and terminals 24 are secured intoa subassembly by retention means 28. To install the subassembly withinhousing 20, a blunt, conical shaped nose cone (not shown) is placed atopa terminal 24. The nose cone includes a base portion substantially thesame diameter as terminal 24 and a conical or tapered end spaced apartfrom the base end and extending away from array 22. The tapered end ofthe nose cone has a terminus that is smaller in diameter than D₁′. Oneend of unstretched housing 20 (shown in FIG. 3) is disposed about thetapered end of the nose cone and housing 20 is then drawn over array 22.As housing 20 is drawn over the array 22, it is stretched so as toaccommodate array 22 and assumes the configuration shown in FIG. 2. Whenstretched to accommodate array 22, housing 20 shrinks in length about 8%as compared to its length before it is radially stretched to accommodatearray 22. Once the housing 20 is stretched about the arrestercomponents, the remaining steps in the assembly process of arrester 20are performed in the following order.

The arrester module is primed with a low viscosity neutral cure siliconeRTV. The primer cure is accelerated at a temperature of between 100 and200° C. Before the housing is applied, a lubricating film of neutralcure RTV is applied, which bonds the housing to the arrestor module. TheRTV can be cured at an accelerating temperature, although this notnecessary. The remaining assembly steps are comparable to those known inthe art of surge arresters.

Referring now to FIG. 5, shed 36 is shown in profile both in theas-molded, unstretched configuration, referred to generally by referencenumeral 66, and its post-stretched configuration 68. As previouslynoted, the ends 42 of shed 36 remains in substantially the same radialposition with reference to housing axis 21 even though the inner andouter surfaces 32, 34 of core 30 are moved radially outward substantialdistances. In the stretched configuration 68, upper surface 38 generallycomprises three interconnected curved surfaces 61-63, curved surface 61and 63 being generally concave while curved surface 62, which isintermediate between surfaces 61 and 63, is generally convex. Thestretched configuration is a function of relative volumes of theunstretched upper and lower portions of each shed.

The present shedded elastomeric housing provides superior performanceand costs less to manufacture than many previously known housings. Costsavings are realized because the perpendicular sheds of the presentinvention are much easier to demold during the manufacturing process.The ease of demolding allows the sheds on the present housing to besignificantly thinner, requiring the use of less material. Quality isalso improved both in the housing itself and its performance. Housingquality is improved because the simpler molded shape results in a lowerdefect rate in molded parts.

Performance is improved because the elastomeric housing can conform toirregularities in the array, particularly if it is used in conjuctionwith a silane surface treatment and/or a silicone RTV material. Thesilane surface treatment and/or silicone RTV material acts to bond thepresent housing to the array so as to prevent the ingress of moisturetherebetween and also functions as a lubricant and void-filling compoundduring the insertion of the arrester module. The present method isadvantageous over conventional methods of molding a housing over anarray, as this molding process requires lower viscosity, less desirablesilicones compounds so as to avoid shifting of the array due to highforces that are imposed during molding. Other suitable bonding agentsinclude silane primers, silicone grease, silicone spray, and similarsubstances, but it is preferred to use substances that provide a bondedinterface.

Ability to perform under operating conditions is affected by the qualityof the interface between the housing and the array. A good measure ofperformance can be made using MultiStress techniques commonly applied onpolymeric insulators and arresters, such as the Italian National utility(ENEL) procedure DY1009 or the IBC procedure EC1109 (1992). Adequateperformance per the ENEL procedure has been achieved due solely to thepressure exerted on the interface due to the level of stretch, providedthat the interface is substantially air free or that air pockets arelarge enough and controllable positioned so as to avoid creatingunacceptable high localized dielectric stresses. The degree offlexibility of the housing depends on the material selected and on theanticipated voltage level. Adequate performance has been demonstrated onan arrestor product having an air-filled open-weave fiberglass cagesimilar to that described in U.S. Pat. No. 5,043,838. Good performancehas also been demonstrated on arrester products using a silicone greasesubstantially air displacing interface on arresters constructed asdescribed in U.S. Pat. No. 4,656,555 and the copending applicationmentioned above. The best performance has been achieved using asubstantially bonded interface and the arrester module construction asdescribed in our copending application. Adequate bonding has beenachieved using a neutral cure RTV silicone compound at the interfacebetween the housing and the array. As discussed above, this materialalso lubricates the housing during placement of the housing over thearray. Further improvements have been noted when the resin coatedmodules have been primed with either a silane-based primer or aspray-on, cured RTV coating similar to those commonly used to coat highvoltage ceramic insulators.

While preferred embodiments of the invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the spirit and teachings of the invention. Theembodiments described herein are exemplary only, and are not limiting.Many variations and modifications of the invention and apparatusdisclosed herein are possible and are within the scope of the invention.Accordingly, the scope of protection is not limited by the descriptionset out above, but is only limited by the claims which follow, thatscope including all equivalents of the subject matter of the claims.

What is claimed is:
 1. An elastomeric housing for electrical apparatus,comprising a deformable shedded sleeve having a central axis andcomprising a tubular core portion with a central bore and a plurality ofaxially-spaced sheds extending from said core; wherein: said core isunstretched when formed and is stretched when an electrical device isinserted in the central bore to form an electrical apparatus, and saidsleeve has a first configuration when said core is unstretched and asecond configuration when said core is stretched; said sheds extend fromsaid core at a first angle relative to said axis when said sleeve is insaid first configuration and extend from said core at a second anglerelative to said axis when said sleeve is in said second configuration;said sheds extend substantially perpendicularly from said core when saidsleeve is in said first configuration and said sleeve assumes saidsecond configuration when said core is stretched radially outwardly; andwhen said sleeve is in said first configuration, said sheds include anupper surface that joins said core in a first shoulder having a radiusof curvature R₁ and a lower surface that joins said core in a secondshoulder having a radius of curvature R₂, with R₁ being grater than R₂.2. The elastomeric housing according to claim 1 wherein when said sleeveis in said second configuration, said sheds extend downwardly from saidcore at an angle within the range of approximately 10 to 45° as measuredfrom a plane that is perpendicular to said axis.
 3. The housingaccording to claim 2 wherein R₁ is at least twice as large as R₂ whensaid sleeve is in said first configuration.
 4. The housing according toclaim 2 wherein said sleeve has substantially the same overall diameterin said first and said second configurations.
 5. The housing accordingto claim 4 wherein in said first configuration, said central bore has adiameter equal to D₁ and wherein in said second configuration said borehas a diameter equal to D₂, where D₂ is greater than D₁.
 6. The housingaccording to claim 2 wherein said sheds include a first end disposed ata predetermined radial position relative to said axis and a second endattached to said core portion, and wherein when said sleeve is in saidsecond configuration, said sheds include a generally frustoconical uppersurface and a convex portion on said upper surface between said firstand said second ends.
 7. The housing according to claim 1 wherein saidsheds include an end disposed at predetermined radial and axialpositions when said sleeve is in said first configuration and whereinwhen said shed is deformed to said second configuration, said shed endsare disposed in axial direction away from said first predeterminedposition but remain at said first predetermined radial position.
 8. Ahousing for electrical apparatus, comprising: an elastomeric sleevehaving a tubular core portion with a central axis and a central bore andhaving a plurality of sheds radiating from said core portion; whereinsaid sheds have ends disposed at a predetermined radial distance fromsaid axis; wherein said sheds include an upper surface comprising afirst frustoconical surface segment joined to said core portion in anupper shoulder having a radius of curvature R₁ and a secondfrustoconical surface segment joined to said first frustoconical surfacesegment at a first transition point T₁; and wherein said sheds include alower surface comprising a third frustoconical surface segment joined tosaid core portion in a lower shoulder having a radius of curvature R₂that is less than R₁ and fourth frustoconical surface segment joined tosaid third frustoconical surface segment at a second transition point T₂that is radially closer to said axis than T₁.
 9. The housing accordingto claim 8 wherein said first frustoconical surface segment tapersdownwardly from said upper shoulder to said first transition point at anangle α₁ and wherein said second frustoconical surface segment tapersdownwardly from said first transition point toward said end of said shedat an angle α₂, wherein α₂ is less than α₁.
 10. The housing according toclaim 9 wherein said third frustoconical surface segment tapers upwardlyfrom said lower shoulder to said second transition point at an angle α₃and wherein said fourth frustoconical surface segment tapers upwardlyfrom said second transition point to said end of said shed at an angleα₄ that is less than α₃.
 11. The housing according to claim 10 whereinα₁ is at least twice as great as α₂.
 12. The housing according to claim10 wherein α₁ is at least four times as great as α₂.
 13. The housingaccording to claim 10 wherein α₃ is substantially equal to α₄.
 14. Thehousing according to claim 10 wherein α₂ is at least twice as large asα₄.
 15. The housing according to claim 10 wherein first frustoconicalsurface segment intersects said upper shoulder at a third transitionpoint and said third frustoconical surface segment intersects said lowershoulder at a fourth transition point, and wherein said fourthtransition point is radially closer to said axis than said thirdtransition point.
 16. The housing according to claim 10 wherein R₁ is atleast twice as large as R₂.
 17. The housing according to claim 8 whereinsaid sleeve is deformable from a first configuration when said core isunstretched to a second configuration when said core is stretchedradially outwardly, said ends of said sheds being lower when said sleeveis in said second configuration compared to first configuration.
 18. Thehousing according to claim 17 wherein when said sleeve is in said secondconfiguration, said ends of said sheds remain at said predeterminedradial distance from said axis.
 19. An elastomeric housing forelectrical apparatus, comprising: a deformable shedded sleeve having acentral axis and comprising a tubular core with a central bore having aninside diameter and a plurality of axially-spaced sheds having upper andlower surfaces and radiating from said core in a first configurationwhen said core is unstretched, wherein said sheds extend substantiallyperpendicularly from said axis when said sleeve is in said firstconfiguration; said sleeve being deformable from said firstconfiguration when said core is unstretched to a second configurationwhen an electrical device is inserted in the central bore to form anelectrical apparatus in which said sheds assume a downwardly extendingposition and said upper surface of said shed is generally frustoconical;and when said sleeve is in said first configuration, said shedsincluding an upper surface that joins said core in a first shoulderhaving a radius of curvature R₁ and a lower surface that joins said corein a second shoulder having a radius of curvature R₂, with R₁ greaterthan R₂.
 20. The elastomeric housing according to claim 19 wherein whensaid sleeve is in said first configuration, each of said upper and lowersurfaces includes at least one frustoconical portion.
 21. Theelastomeric housing according to claim 20 wherein when said sleeve is insaid first configuration, said first frustoconical portion intersects aplane perpendicular to said axis at an angle of approximately 2.5°. 22.The elastomeric housing according to claim 20 wherein each of said upperand lower surfaces includes two concentric frustoconical portions. 23.The elastomeric housing according to claim 22 wherein when said sleeveis in its first configuration, said second frustoconical portionintersects a plane perpendicular to said axis at an angle of less thanapproximately 2.5°.
 24. The elastomeric housing according to claim 23wherein R₁ is at least twice as large as R₂.
 25. The elastomeric housingaccording to claim 24 wherein each of said upper and lower surfacesincludes a first frustoconical portion, said upper first frustoconicalportion intersecting said first shoulder at a first upper transitionpoint and said lower first frustoconical portion intersecting saidsecond shoulder at a first lower transition point, said first uppertransition point being at a greater radial distance from said axis thansaid first lower transition point.
 26. The elastomeric housing accordingto claim 25 wherein each of said upper and lower surfaces furtherincludes a second frustoconical portion, said upper second frustoconicalportion intersecting said upper first frustoconical portion at a secondupper transition point and said lower second frustoconical portionintersecting said lower first frustoconical portion at a second lowertransition point, said second upper transition point being at a greaterradial distance from said axis than said second lower transition point.27. The elastomeric housing according to claim 19 wherein each shed hasan upper surface and a lower surface and wherein when said sleeve is insaid second configuration, said upper surface includes first and secondcircumferentially concave portions and a first circumferentially convexportion therebetween.
 28. The elastomeric housing according to claim 19wherein said sheds comprise radially extending members having outeredges, said radially extending members decreasing in thickness towardtheir outer edges.
 29. The elastomeric housing according to claim 28wherein in both said first and second configurations, said outer edgesof said sheds remain at substantially the same radial position relativeto said axis.